According to the specification of the DVD-RW (Standard ECMA-338-80 mm (1.46 Gbytes per side) and 120 mm (4.70 Gbytes per side) DVD Re-recordable Disk (DVD-RW)), the push-pull signal obtained from this type of optical recording medium includes a land pre-pit (LPP) signal and a wobble signal. The LPP signal consists of a sequence of peaks. The wobble signal is a sinusoidal signal. The peaks of the LPP signal are quite narrow in comparison with the period of the wobble, and are superimposed on the wobble. Only one peak occurs for each wobble period and its phase relative to the wobble is 270° with a tolerance of ±10°. Most of the wobble periods do not have a peak. Only some of them, minimum one and maximum three of 16 periods, can have a peak. This rule is defined by the specific data coding of LPP data. In every pair of Sync frames (16 wobble periods), there is a set of three bits, which determine the presence of peaks according to on/off modulation. The set is either at the beginning of the first frame or at the beginning of the second one. The first bit of the set is a frame Sync bit, so it is always 1. The second bit is a sector Sync bit, so it is always 1 in the first pair of Sync frames of the sector (26 frames). The third bit is a data bit. Only in the first pair of Sync frames the third bit is not a data bit, but is related to the location of the set, namely 1 for the first frame, 0 for the second one.
For writing to the disk a DVD recorder has to be locked to the wobble signal and to decode the LPP data. Unfortunately, several interfering disturbances degrade the quality of the push-pull signal:    1.) Tracking error signal: This signal is located in the lower part of the frequency spectrum. Therefore, it does not disturb the wobble signal, which has higher frequency. However, it interferes with the LPP signal, specifically the sector Sync word, which has a very low frequency.    2.) Residual data signal HF: When the focus of a light beam used for reading is not perfectly located on the track, the four output currents of a photodetector used for detecting the light reflected from the disk are not perfectly balanced. When the push-pull signal is produced, a part of the data signal HF passes. This data signal HF does not disturb the wobble signal, which has lower frequency, but it disturbs the LPP signal.    3.) Disk speed variations: The eccentricity, the servo control loop and the constant angular velocity (CAV) cause changes of the disk speed. Therefore, the wobble signal is not a pure sinus, but it is frequency modulated according to the disk speed with a corresponding spreading of the bandwidth.    4.) Interference of adjacent tracks: The wobble signal of adjacent tracks overlaps with the wobble signal of the current track, generating an amplitude modulation of the wobble signal and violating the abovementioned phase relationship. Therefore, the physical tolerance of ±10° is dramatically increased.
It is an object of the invention to provide a method for recovering land pre-pits with a decoder from a signal obtained from an optical recording medium, which overcomes the above problems.